Gas chromatography is essentially a physical method of separation in which constituents of a vapor sample in a carrier gas are adsorbed or absorbed and then desorbed by a stationary phase material in a column. Interactions between this stationary phase material and the various components of the sample—which differ based upon differences among partition coefficients of the components—cause the sample to be separated into the respective components. At the end of the column, the individual components are more or less separated in time. Detection of the gas provides a time-scaled pattern, typically called a chromatogram, that, by calibration or comparison with known samples, indicates the constituents, and the specific concentrations thereof, which are present in the test sample. An example of the process by which this occurs is described in U.S. Pat. No. 5,545,252 to Hinshaw.
Often, the analytes to be measured are retained by and concentrated on an adsorbent in a sample tube. Once the analytes are collected in the sample tube, the tube is then transported to a thermal desorption unit, where the tube is placed in the flow path of an inert gas, such as helium or nitrogen. The tube is subsequently heated, thereby desorbing the analytes, and the carrier gas sweeps the analytes out of the tube. In some cases, a trap is located downstream of the sample tube in order to further pre-concentrate the analytes, and occasionally, remove moisture therefrom, prior to introducing the sample into the chromatographic column. One example of such a trap is an adsorbent trap, usually cooled to a sub-ambient temperature, which may simply be another sorbent tube with a suitable adsorbent material. The adsorbent trap adsorbs the analytes as the sample gas first passes through the tube. The analytes are then subsequently desorbed into the chromatographic column from the trap, usually by heating, for subsequent separation and analysis as discussed above. Typically, either the column is directly coupled to a sorbent tube in the thermal desorption unit or the unit is connected directly to the column via a transfer line, such as, for example, via a length of fused silica tubing.
One common application of chromatographic analysis using a thermal desorption unit is to determine the constituents of a particular environment. For example, it is often desired to detect the amount of volatile organic compounds (VOCs) present in a certain sample of air. One way of doing this is by first transporting a sample tube packed with an adsorbent material into the environment to be tested, and allowing the VOCs in the air to migrate into the tube through natural diffusion, typically termed “diffusive” or “passive sampling.” Alternatively, the VOCs may be collected by drawing a sample of gas (typically ambient air) through such a tube using a small vacuum pump, commonly referred to as “pumped sampling.” In each case, the analytes to be measured (i.e., the VOCS) are retained by and concentrated on the adsorbent as the air passes through the tube.
Once the VOCs are collected in this fashion, the sample tube is then transported to the thermal desorption unit, where the tube is placed in the flow path of an inert gas, such as helium or nitrogen. The sample tube is subsequently heated, thereby desorbing the analytes, and the carrier gas sweeps the VOCs out of the tube and into a second trap, typically cooled to sub-ambient temperatures, or the chromatographic column. Typically, either the chromatographic column is directly coupled to a sorbent tube or the sub-ambient trap in the thermal desorption unit or the unit is connected directly to the column via a transfer line, such as, for example, via a length of fused silica tubing.
Some thermal desorption apparatus are automated thermal desorption apparatus or units that automate the processing and handling of sample tubes. For example, the TurboMatrix 650 ATD available from PerkinElmer is an automated analytical thermal desorber including a carousel, a robot arm, a capping/decapping station, and a sampling station. In use, the robot arm automatically collects a sample tube from the carousel and transports the sample tube to the capping/decapping station where end caps are removed from the sample tube. The robot arm then places the uncapped sample tube in the sampling station. In the sampling station, the sample tube is heated to desorb an analyte from the sample tube as described above. Thereafter, the robot arm returns the sample tube to the capping/decapping station (where the sample tube is recapped) and then the carousel.